I agree with Elizabeth that it intuitively feels like SD fits more with things like exercise and infection than things like toe-stubbing. I did not have a ready-made answer before you asked the important question “why?,” and I think that means you should discount this somewhat as rationalization. With that caveat, here goes!
First, let’s look at some of the underlying physiological impacts of these potentially “good stressors.”
Exercise: tear and repair of muscle fibers results in altered morphology
Infection: repeated exposure results in death of dysfunctionally reactive immune cells
Fasting: hormone changes in response to change in metabolism (I’m handwaving to the max!)
Extreme temperatures: one impact is that elevated body temperature causes the release of heat shock proteins, which directly impact immune system function, such as by increasing MHC class I loading (this is an important role of fever in fighting off infections). It also affects the material properties of proteins, which could impact things like flexibility but also enzyme reaction rates (though not necessarily in a straightforward way, since the body attempts to create homeostasis and there are all kinds of feedback loops).
Let’s start not with a stubbed toe, but with breaking a bone. What does this lack that excludes it from the potentially “good stressor” category, or what does it include that puts it in the “bad stressor” category?
Clearly, bone breaking is incredibly, durably painful. It potentially permanently alters the physical shape of bones in a way that can easily destroy their ability to naturally regain their original shape. Since the body depends on the skeleton to operate muscles effectively, strength, flexibility, and dexterity may be impacted as well. Furthermore, bone breakage may cause large-scale tearing of soft tissue and generate a massive infection site, potentially concomitant with tearing of blood vessels and solid organ puncture. These large-scale physical impacts are what put bone breaking into the “bad stressor” category, and I think it’s safe to say nobody in their right mind would hope to find some countervailing positive effect that’s worth this massive cost.
As we scale down the magnitude and duration of pain, the extent of physical tissue destruction, the destruction of the skin’s barrier against infection, we overall scale down many of the things that put an acute stressor into the “bad” category, potentially allowing any benefits it may have to become relevant.
Since sleep deprivation entails no tissue destruction, except insofar as it impacts synaptogenesis or axonal pruning (a field of study where our knowledge is quite patchy!), we can’t start from a physical assumption that it’s obviously bad.
Now, let’s compare with the stubbed toe, and also with exercise.
Exercise, unlike a stubbed toe, is a natural consequence of human activity that’s necessary for life. As such, if there are inherently damaging aspects of exercise, we can expect that evolution has had an enormous amount of time to work on mitigating them. Biomechanical cues impacting development are all over the place in the body. Because it’s metabolically expensive to maintain unnecessary girth, we can interpret the body’s ability to respond to cues saying “grow more and bigger cells here, we’re obviously using them” as a helpful regulatory feature, one that might be evolutionarily advantageous.
By contrast, we also expect nature to put highly aversive pain signals in place where they’re most useful, since pain comes with costs. A stubbed toe isn’t a signal that we’re “using that toe,” but a signal to adjust how we’re walking and of potential injury. There’s again a straightforward biomechanical interpretation to be found.
Although sleep pressure is often aversive, this aversion comes from an entirely different physical system, and it’s not clear that it means the same thing. In addition, I can control my sleep aversion to an extent that’s impossible without drugs when it comes to pain. The controllability, modest extent, origin from a source other than tissue trauma, and context as part of the normal pattern of behavioral variation in organisms on a day-to-day basis set sleep deprivation apart from a stubbed toe and in the category of exercise, exposure to pathogens and chemicals, fasting, and temperature variation.
None of this necessarily means that sleep deprivation (or any of these other “relatively normal, non-injurious” interventions) is good for you.
If they were, though, we might offer a unified meta-reason. In general, all these sources of variation may be ways to supply useful regulatory cues to the body.
Now, let’s zoom in on sleep. Even when we look specifically at sleep research, as opposed to work-related fatigue, we have a confounder that I suspect may not be adequately addressed.
This is light. Where I am, it’s 1:20 AM, and I have the slights on blazing bright while staring into my computer screen. In addition to my wakefulness, my body us receiving a cue about when light is available. Returning to the evolutionary biology lens, this would be ancestrally correlated with my ability to hunt, my concerns about predators, social activity, the season, and so on. Potentially, more sleep goes with more darkness goes with less abundant food and social opportunity goes with hibernation. More wakefulness would be ancestrally associated with more abundance and opportunity and fewer predation threats. This is aside from the fact that we can actually do more while awake, which may have additional benefits if we use our time wisely.
So this is a candidate for a reason why sleep deprivation could have some benefits. It could either be entirely due to light exposure during additional hours of wakefulness, or to wake and sleep being used as proxies for opportunities while awake. The idea that light cues and circadian rhythms are important controllers of our behavior and experience are already well-established (light therapy is a depression treatment, as is sleep deprivation).
A second possibility is that sleep deprivation could, on its own, diminish bad forms of mental and emotional stress. Hear me out! Usually, when people are sleep deprived, it’s because they’re trying to cope with some very stressful circumstances—a new baby, a demanding job, an upcoming exam, jet lag, a war or other stressful environment. So it is associated with high stress, and often perceived to be at least a contributing cause of it.
However, it’s easy to picture how sleeping for fewer hours, in these situations, is often a way to reduce and manage that stress. You are able to take care of activities that need to be done with those additional waking hours, thereby removing sources of worry and discomfort that would otherwise bother you while you’re trying to sleep.
So imagine if regular folks, without any extreme stressors, chose to take a couple extra hours awake to take care of random lingering sources of discomfort or stress. A couple extra hours per day could easily be the difference between “I have only just enough time to get everything done” and “I have plenty of time to relax and make improvements to the conditions of my life.” In this case, we’d see sleep deprivation reducing people’s stress over time.
Note that neither of these hypotheses are out of the bounds of our familiar experience. Plenty of people have said “oh, if only I had an extra hour or two in my day.” We already use both sleep deprivation and light therapy as depression treatments, and they fit naturally into the category of environmental and behavioral cues regulating physiological processes.
In fact, the real mystery is why sleep deprivation should be bad for us, not why it should be neutral-to-good for us. Not all animals sleep, and amount of sleep isn’t really correlated with cognitive capacity as far as I remember (I’m being lazy and not looking up the relevant study, sorry!). We also have a great alternative explanation for why animals sleep: to reduce metabolic demands during times of high threat and low light, as in hibernation.
One physiological reason to think sleep might in fact be important is that the brain, like other organs, may need to physically alter its morphology in order to accomplish maintenance tasks. These might include learning-central tasks like synaptogenesis, but it also might just entail things like clearance of fluids. I think it’s fairly well-established that this does happen during sleep. In addition, I understand that it’s not just the physical connectivity of synapses, but also the overall flow of electrical potential throughout the brain as a whole, that governs thought. So it’s plausible that sleep is necessary to perform essential maintenance tasks that degrade the physical ability of the brain to function properly if they aren’t done. Alternatively, it might be that these maintenance tasks will intrude into the waking hours, disrupting cognition in an unpleasant fashion, if they don’t happen during sleep.
I agree with Elizabeth that it intuitively feels like SD fits more with things like exercise and infection than things like toe-stubbing. I did not have a ready-made answer before you asked the important question “why?,” and I think that means you should discount this somewhat as rationalization. With that caveat, here goes!
First, let’s look at some of the underlying physiological impacts of these potentially “good stressors.”
Exercise: tear and repair of muscle fibers results in altered morphology
Infection: repeated exposure results in death of dysfunctionally reactive immune cells
Fasting: hormone changes in response to change in metabolism (I’m handwaving to the max!)
Extreme temperatures: one impact is that elevated body temperature causes the release of heat shock proteins, which directly impact immune system function, such as by increasing MHC class I loading (this is an important role of fever in fighting off infections). It also affects the material properties of proteins, which could impact things like flexibility but also enzyme reaction rates (though not necessarily in a straightforward way, since the body attempts to create homeostasis and there are all kinds of feedback loops).
Let’s start not with a stubbed toe, but with breaking a bone. What does this lack that excludes it from the potentially “good stressor” category, or what does it include that puts it in the “bad stressor” category?
Clearly, bone breaking is incredibly, durably painful. It potentially permanently alters the physical shape of bones in a way that can easily destroy their ability to naturally regain their original shape. Since the body depends on the skeleton to operate muscles effectively, strength, flexibility, and dexterity may be impacted as well. Furthermore, bone breakage may cause large-scale tearing of soft tissue and generate a massive infection site, potentially concomitant with tearing of blood vessels and solid organ puncture. These large-scale physical impacts are what put bone breaking into the “bad stressor” category, and I think it’s safe to say nobody in their right mind would hope to find some countervailing positive effect that’s worth this massive cost.
As we scale down the magnitude and duration of pain, the extent of physical tissue destruction, the destruction of the skin’s barrier against infection, we overall scale down many of the things that put an acute stressor into the “bad” category, potentially allowing any benefits it may have to become relevant.
Since sleep deprivation entails no tissue destruction, except insofar as it impacts synaptogenesis or axonal pruning (a field of study where our knowledge is quite patchy!), we can’t start from a physical assumption that it’s obviously bad.
Now, let’s compare with the stubbed toe, and also with exercise.
Exercise, unlike a stubbed toe, is a natural consequence of human activity that’s necessary for life. As such, if there are inherently damaging aspects of exercise, we can expect that evolution has had an enormous amount of time to work on mitigating them. Biomechanical cues impacting development are all over the place in the body. Because it’s metabolically expensive to maintain unnecessary girth, we can interpret the body’s ability to respond to cues saying “grow more and bigger cells here, we’re obviously using them” as a helpful regulatory feature, one that might be evolutionarily advantageous.
By contrast, we also expect nature to put highly aversive pain signals in place where they’re most useful, since pain comes with costs. A stubbed toe isn’t a signal that we’re “using that toe,” but a signal to adjust how we’re walking and of potential injury. There’s again a straightforward biomechanical interpretation to be found.
Although sleep pressure is often aversive, this aversion comes from an entirely different physical system, and it’s not clear that it means the same thing. In addition, I can control my sleep aversion to an extent that’s impossible without drugs when it comes to pain. The controllability, modest extent, origin from a source other than tissue trauma, and context as part of the normal pattern of behavioral variation in organisms on a day-to-day basis set sleep deprivation apart from a stubbed toe and in the category of exercise, exposure to pathogens and chemicals, fasting, and temperature variation.
None of this necessarily means that sleep deprivation (or any of these other “relatively normal, non-injurious” interventions) is good for you.
If they were, though, we might offer a unified meta-reason. In general, all these sources of variation may be ways to supply useful regulatory cues to the body.
Now, let’s zoom in on sleep. Even when we look specifically at sleep research, as opposed to work-related fatigue, we have a confounder that I suspect may not be adequately addressed.
This is light. Where I am, it’s 1:20 AM, and I have the slights on blazing bright while staring into my computer screen. In addition to my wakefulness, my body us receiving a cue about when light is available. Returning to the evolutionary biology lens, this would be ancestrally correlated with my ability to hunt, my concerns about predators, social activity, the season, and so on. Potentially, more sleep goes with more darkness goes with less abundant food and social opportunity goes with hibernation. More wakefulness would be ancestrally associated with more abundance and opportunity and fewer predation threats. This is aside from the fact that we can actually do more while awake, which may have additional benefits if we use our time wisely.
So this is a candidate for a reason why sleep deprivation could have some benefits. It could either be entirely due to light exposure during additional hours of wakefulness, or to wake and sleep being used as proxies for opportunities while awake. The idea that light cues and circadian rhythms are important controllers of our behavior and experience are already well-established (light therapy is a depression treatment, as is sleep deprivation).
A second possibility is that sleep deprivation could, on its own, diminish bad forms of mental and emotional stress. Hear me out! Usually, when people are sleep deprived, it’s because they’re trying to cope with some very stressful circumstances—a new baby, a demanding job, an upcoming exam, jet lag, a war or other stressful environment. So it is associated with high stress, and often perceived to be at least a contributing cause of it.
However, it’s easy to picture how sleeping for fewer hours, in these situations, is often a way to reduce and manage that stress. You are able to take care of activities that need to be done with those additional waking hours, thereby removing sources of worry and discomfort that would otherwise bother you while you’re trying to sleep.
So imagine if regular folks, without any extreme stressors, chose to take a couple extra hours awake to take care of random lingering sources of discomfort or stress. A couple extra hours per day could easily be the difference between “I have only just enough time to get everything done” and “I have plenty of time to relax and make improvements to the conditions of my life.” In this case, we’d see sleep deprivation reducing people’s stress over time.
Note that neither of these hypotheses are out of the bounds of our familiar experience. Plenty of people have said “oh, if only I had an extra hour or two in my day.” We already use both sleep deprivation and light therapy as depression treatments, and they fit naturally into the category of environmental and behavioral cues regulating physiological processes.
In fact, the real mystery is why sleep deprivation should be bad for us, not why it should be neutral-to-good for us. Not all animals sleep, and amount of sleep isn’t really correlated with cognitive capacity as far as I remember (I’m being lazy and not looking up the relevant study, sorry!). We also have a great alternative explanation for why animals sleep: to reduce metabolic demands during times of high threat and low light, as in hibernation.
One physiological reason to think sleep might in fact be important is that the brain, like other organs, may need to physically alter its morphology in order to accomplish maintenance tasks. These might include learning-central tasks like synaptogenesis, but it also might just entail things like clearance of fluids. I think it’s fairly well-established that this does happen during sleep. In addition, I understand that it’s not just the physical connectivity of synapses, but also the overall flow of electrical potential throughout the brain as a whole, that governs thought. So it’s plausible that sleep is necessary to perform essential maintenance tasks that degrade the physical ability of the brain to function properly if they aren’t done. Alternatively, it might be that these maintenance tasks will intrude into the waking hours, disrupting cognition in an unpleasant fashion, if they don’t happen during sleep.